1. Field of Invention
The present invention relates to a magnetoresistive random access memory (MRAM). More particularly, the present invention relates to an upside-down memory structure of MRAM.
2. Description of Related Art
Magnetoresistive random access memory (MRAM) is a type of non-volatile memory with fast programming time and high density. An MRAM cell has two ferromagnetic layers separated by a non-magnetic layer. Information is stored as directions of magnetization vectors in the two ferromagnetic layers.
FIG. 1 is a schematic view of a conventional MRAM of a standard structural design. As illustrated in FIG. 1, an MRAM 100 has a plurality of MRAM cells 106 placed on intersections of “M” sense lines 102 and “N” word lines 104, thus forming an N×M matrix. The MRAM cells 106 of one sense line 102, which present memory bits of the MRAM 100, are electrically connected in series between a ground voltage (GND) and a transistor 112 for row selection. The transistor 112 is further electrically connected to a supply voltage (VDD) in series through two transistors 114 and 116 for sense column selection and sense current source, respectively.
The MRAM cells 106 are accessed with sense currents which are generally controlled by current mirrors. However, in the conventional MRAM of the standard structural design, the current mirrors are unstable because they are relative to the high voltage (VDD), which tends to have many variations. Moreover, the sense lines 102 are connected together with only one drawing current, and when one sense line becomes active, the other sense lines load the active sense line. That is, in order to obtain the signal from the active sense line 102, other sense lines connected together have to be charged to the value set by the active sense line. The RC time constant is thus increased, and the settling time for the active sense line requires additional time for both READ and WRITE cycles, worsening the performance of MRAM.